1. Field of the Invention
The present invention relates to plasma generators and their applications in plasma chemistry and technology. In particular, to the provision of a method of using a fluidized bed with low-temperature plasma without the use of a grid, the improvement of the efficiency and lifetime of reactors, and the design of electrodes in a Tornado Sliding Arc Plasma Generator (TSAPG).
2. Description of the Related Technology
Improving the efficiency of the operation of a fluidized bed remains an important technological goal, owing to the significant economic benefits that result in almost every sector of the economy.
Physical processes that utilize fluidized beds include drying, mixing, granulation, coating, heating and cooling. All these processes take advantage of the excellent mixing capabilities of the fluidized bed. Good solids mixing gives rise to good heat transfer, temperature uniformity and ease of process control. One of the most important applications of the fluidized bed is to the drying of solids. Fluidized beds are currently used commercially for drying such materials as crushed minerals, sand, polymers, pharmaceuticals, fertilizers and crystalline products.
Fluidized beds are often used to cool particulate solids following a reaction. Cooling may be by fluidizing air alone or by the use of cooling water passing through tubes immersed in the bed.
Other examples of the application of fluidized bed technology to different kinds of chemical reaction are ethylene hydrogenation, sulfide ore roasting, combustion, and hydrocarbon cracking. Reasons for using fluidized beds are the substantially uniform temperature inside the bed, ease of solid handling, and good heat transfer that they provide.
A new approach to the production of Vinyl Acetate Monomer (VAM) is to use a fluidized-bed process in which gas phase reactants are contacted continuously over (small-sized) supported catalytic particles under fluidized conditions.
Fluidized beds are also used as sorters in the food processing industry. This technology uses a mobile field separation apparatus that has a dry fluidized bed system with sand as the fluidized medium. The technology can remove all dirt clumps from the lifted product stream, such as from potatoes. The technology could be applied to cleaning field tare from incoming raw food product streams and could be used by industrial processors to replace water flumes that consume significant electrical power and water and require a relatively high degree of maintenance.
Fluidized beds can also be used in gasification systems. The fluid bed converts, for example, biomass waste products into a combustible gas that can be fired in a boiler, kiln, gas turbine or other similar device as a means to convert a portion of the fuel supply to clean, renewable biomass fuel. Gasification is the thermal decomposition of organic matter in an oxygen deficient atmosphere producing a gas composition containing combustible gases, liquids and tars, charcoal, and air, or inert fluidizing gases. Typically, the term “gasification” refers to the production of gaseous components.
A gas distributor is a device designed to ensure that the fluidizing gas is always substantially evenly distributed across the cross-section of the bed. It is an important part of the design of a fluidized bed system. Good design is based on achieving a pressure drop, which is a sufficient fraction of the bed pressure drop. Some distributor designs in common use are (a) drilled plate, (b) cap design, (c) continuous horizontal slots, (d) stand pipe design, and (e) sparge tubes with holes pointing downwards.
Loss of fluidizing gas will lead to collapse of the fluidized bed into a packed bed. If the process involves the generation of heat, then this heat will not be dissipated as well from the packed bed as it was from the fluidized bed.
All parts of the fluidized bed unit are subject to erosion by the solid particles. Heat transfer tubes within the bed or freeboard are particularly at risk and erosion here may lead to tube failure. Erosion of the distributor may lead to poor fluidization and areas of the bed becoming de-aerated. Loss of fine solids from the bed reduces the quality of fluidization and reduces the area of contact between the solids and the gas in the process. In a catalytic process this generally results in lower conversion.
In addition, reactors employing a grid for the generation of plasma are also subject to erosion by contact with solid particulates. Also, generation of plasma using a grid is less energy efficient than other methods of plasma generation.
In a fluidized bed combustion chamber, known as a spouted bed reactor, a cone shaped hopper is continuously fed with solid particles. The solid particles are suspended briefly and processed in an axial flow of gas originating from the bottom or apex of the cone. One disadvantage of the spouted bed reactor is the instability of the axial gas flow. The solid particles fall out of suspension easily due to turbulence and accumulate in the narrower bottom portion of the cone. In addition, the bottom entry tube provides only an axial gas flow velocity component. Thus, there is no orthogonal gas flow velocity component to assist in distributing the solid particles throughout the cone shaped reactor. Consequently, the mixing of solid particles with gas, and the interaction among the solid particles, are relatively poor. The poor mixing and non-uniform particle distribution result in a relatively low efficiency of combustion and/or gasification.
In order to improve the distribution of solid particulates throughout a cone shaped reactor and, thereby, minimize inefficiencies produced by non-ideal particle distributions, it is known to utilize a circumferential flow of gas whose direction is orthogonal to the axial gas flow. The axial and circumferential gas flows may preferably be adjusted to produce a vortex in the conical reactor.
U.S. Pat. No. 5,486,269, for example, describes an inverted conical reactor, suitable for coal gasification, which uses a tangential flow of air to achieve a vortex flow pattern.
None of these devices and methods, however, provides fluidization of solid particulates with optional plasma energy input and without employing a grid. Therefore, there remains a need for a fluidization reactor with optional plasma energy input that does not employ a grid.
Another application of plasma technology is thermal spray deposition. A plasma jet generated by a plasma generator accelerates and melts particles of the material to be deposited on the substrate. This technology is widely used for the development of hard, corrosion-resistant, thermal barriers and other types of coatings. DC plasma torches used in such plasma generators have a very limited lifetime because of electrode erosion.
Another application of plasma technology is thermal waste destruction. In this case plasma generators are used as sources of high temperature and/or chemically active plasma for waste treatment. Also, in some cases plasma generators are used as reactors for waste destruction. In all these cases, use of cheap and effective DC or AC plasma generators with open (non-insulated) electrodes is limited by the very limited lifetime of the electrodes.
Other applications of plasma technology include different plasma chemical processes (decomposition of chemical solutions, oxidation and reduction of different metals, production of nano-particles, surface treatment and sterilization, and so on), welding, cutting, etc. In all cases of plasma technology application the key problem related to the use of cheap and effective DC and AC plasma generators and plasma reactors, is the limited lifetime of the electrodes.
Additionally, reverse vortex (tornado) sliding (or gliding) arc plasma generators (TSAPG) with electrodes for DC or AC current have been recently developed. Electrodes for TSAPG in current use have complex shapes (spiral or ring). These electrodes are typically submerged in the plasma generator volume, which results in gas flow disturbance and electrode overheating. Despite the recent development of these generators, the problems with the overall lifetime of the plasma generators and the efficiency of the energy use have not been completely solved. An optimal electrode design has not yet been developed.
Additionally, flame stabilization of lean and super lean fuel and air mixtures for NOx reduction is a key problem for modern gas turbine engines and burners. One of the well-known solutions is utilization of pilot flames, but existing pilot flames generate too much nitrogen oxide. Utilization of a DC plasma torch for flame stabilization in gas turbine engines works well in the combustion process, but its application is restricted by the limited life cycle of the plasma generator electrodes. The electrode design should ensure reliable ignition of the arc, long lifetime of the electrodes and absence of significant disturbances of the reverse vortex flow.
Therefore, there remains a need for an electrode design that provides an extended lifetime of the electrodes and increases the energy efficiency of the system.